High-efficiency diode-pumped lasers

ABSTRACT

Certain practical difficulties are encountered in positioning a pump source comprising very-small-diameter electroluminescent diodes in direct physical contact with an associated laser rod. These difficulties are surmounted by positioning the diodes and the rod in a spaced-apart relationship and imaging the emitting areas of the diodes to positions slightly within the rod. In addition, a major portion of the surface of the rod is coated with a reflective material whereby radiation that enters the rod is multiply reflected therein. In this way the radiant output of the diodes is coupled to the rod in a highly efficient manner by means of a structure that is easily realized.

United States Patent Ostermayer, Jr.

[451 May 16, 1972 [72] Inventor: Frederick William Ostermayer, Jr., New

Providence, NJ.

[73] Assignee: Bell Telephone Laboratories, Incorporated,

Murray Hill, NJ.

[22] Filed: July 13, 1970 [21] Appl. No.: 54,538

Griest ..331/94.5

OTHER PUBLICATIONS Ross, Proc. IEEE, 56, (2), Feb. 1968, pp. 196- 197Primary Examiner-Ronald L. Wibert Assistant Examiner-R. J. WebsterAttorney-R. J. Guenther and Kenneth B. Hamlin [5 7] ABSTRACT Certainpractical difficulties are encountered in positioning a pump sourcecomprising very-small-diameter electroluminescent diodes in directphysical contact with an associated laser rod. These difiiculties aresurmounted by positioning the diodes and the rod in a spaced-apartrelationship and imaging the emitting areas of the diodes to positionsslightly within the rod. In addition, a major portion of the surface ofthe rod is coated with a reflective material whereby radiation thatenters the rod is multiply reflected therein. In this way the radiantoutput of the diodes is coupled to the rod in a highly efiicient mannerby means of a structure that is easily realized.

2 Claims, 6 Drawing Figures PATENTEDMAY 1 6 I972 SHEET 1 IJF 3 FIG.

E a, M m m N 5% y PATE'N'TEIHW 16 1972 v 3.663.893

SHEET 3 [IF 3 FIG. 4A

This invention relates to lasers and more particularly to varioustechniques for achieving a high-efficiency diodepumped solid-statelaser.

BACKGROUND OF THE INVENTION It is known to employ an array of incoherentelectroluminescent diodes to pump a laser rod. Satisfactory operation ofsuch an arrangement depends on efficiently coupling the radiationemitted by the diodes to the rod.

One conventional approach to constructing a diodepumped laserarrangement involves placing the diodes in contact with the surface ofthe laser rod. Alternatively, the diodes may be imbedded in the rod. Inboth of these cases, the close proximity between the diodes and the 'rodinsures that a reasonably high degree of coupling is achieved betweenthe radiant output of the diodes and theabsorption states of the rod.

In practice, however, mechanical difficulties have been encountered inmounting very-smaIl-diameter diodes in actual physical contact with anassociated laser rod. Accordingly, ef.- forts have been directed atpositioning the diodes in aspacedapart relationship with respect to thelaser rod while attempting still to realize the high efficiencyadvantage of close-proximity coupling.

SUMMARY OF THE INVENTION An object of the present invention is animproved diodepumped laser.

More specifically, an object of this invention is a relatively simplediode-pumped laser characterized by high efiiciency and ease offabrication.

Briefly, these and other objects of the present invention are realizedin a specific illustrative embodiment thereof in which images of aplurality of diodes are directed by a reflector to the respectivepositions the diodes would occupy if they were actually mounted in closeproximity to an associated laser rod.

In addition, since in practice the diameters of the respective emittingareas of the diodes are typically smaller (by a factor of 2 or 3) thanthe diameter of the associated rod, the invention also encompassescoating a major portion of the surface area of the rod with a reflectivematerial whereby radiation that enters the rod is multiply reflectedtherein. Such multiple reflections substantially increase the length ofthe interaction path that the radiation traverses in the rod. In thisway, highefficiency coupling in a simple mechanically-realizable laserstructure is achieved.

In accordance with one specific aspect of the present invention, imagingof the diodes is accomplished with a semi-el1iptical cylinder whoseconcave surface is highly reflective. A-

linear array of diodes is disposed along one focal line of the cylinderand the laser rod is positioned with its longitudinal axis parallel toand slightly displaced from the other focal line.

All but a relatively narrow strip of the rod is coated with a reflectivematerial.

In another illustrative embodiment of this invention, twosemi-elliptical cylinders are utilized to image two linear arrays ofdiodes onto two opposed nonreflective strips of an otherwisereflectively-coated laser rod. In other embodiments, three-dimensionalreflectors such as a semi-ellipsoid or a hemisphere are employed toimage the diodes onto a laser rod. In each of these last-mentionedembodiments, the laser rod is coated with high-reflectivity materialexcept for discrete areas corresponding to the positions of the diodeimages. Such large-area coatings enable these structures to exhibitparticularly efficient coupling characteristics.

Accordingly, a principal feature of the present invention is that imagesof pump diodes be directed onto a laser rod whose surface is coated witha material that multiply reflects diode radiation that enters the rod.

BRIEF DESCRIPTION OF THE DRAWING A complete understanding of the presentinvention and of the above and other objects, features and advantagesthereof may be gained from a consideration of the following detaileddescription of several illustrative embodiments thereof presentedhereinbelow in conjunction with the accompanying drawing, in which:

FIG. 1 shows a specific illustrative arrangement made in accordance withthe principles of the present invention;

FIG. 2 is an end view of a portion of the FIG. I arrangement;

FIG. 3 shows another illustrative embodiment of the principles of thisinvention;

FIG. 4A illustrates the radiation pattern of one of the diodes includedin the arrangements shown in FIGS. 1 and 3;

FIG. 4B shows a variant of the arrangements of FIGS. 1 and 3; and

FIG. 5 depicts still another illustrative embodiment made in accordancewith the principles of the present invention.

DETAILED DESCRIPTION vtively, the element 10 is a cylindrical rod(having a longitudinal axis 12) whose ends are flat and parallel to eachother or are convex spherical segments whose centers lie on thelongitudinal axis 12. If the depicted arrangement is intended to operateas an oscillator, one of the ends of the rod is treated in a knownmanner to be completely reflective while the other or output end isadapted to be partially reflective (and partially transmissive) to laserradiation incident thereon, or the rod may be placed between externalreflectors so treated.

The rod 10 shown in FIG. 1 is optically pumped by an array ofelectroluminescent diodes 15 disposed in a linear fashion parallel tothe aforementioned axis 12. Illustratively, the diodes 15 arecommercially-available, dome-shaped, infraredemitting gallium arsenidephosphide junction devices. In a conventional manner (not shown) each ofthe diodes 15 is provided with electrodes for connection to a source(not shown) for forward-biasing the diodes to their emitting condition.

Alternatively, the dome-shaped diodes 15 shown in FIG. 1 may be madeinaccordance with the teachings contained in copending application Ser.No. 816,763, now U.S. Pat. No. 3,593,055, issued July 13, 1971. Inaccordance with that application, reduced-dome-radius diodes are formed,with the result that the conversion efficiency of the devices forgenerating infrared radiation is thereby significantly enhanced.

The diodes 15 and the rod 10 of FIG. 1 are mounted in a base plate 16which is made of any suitable high-heat conductivity material such ascopper. By way of illustration, the lower portions of the dome-toppeddiodes 15 may be positioned in individual receiving cavities (forexample, cylindrical holes) formed in the plate 16. Additionally, therod 10, a substantial portion of whose surface area is covered with alayer 18 whose nature will be specified below, is positioned in alongitudinal slot cut in the plate 16.

Disposed on the plate 16 of FIG. 1 is a metallic block 20, a portion ofwhose bottom facedefines the curved surface of a semi-ellipticalcylinder. The curved surface 22 of the block 20 is treated in any of avariety of ways known in the art to be highly specularly reflective toradiation directed thereat by the diodes 15.

Associated with and definitive of the semi-elliptical surface 22 are twofocal lines 24 and 26. The line 24 extends through and is approximatelycentrally located with respect to the emitting areas of the diodes 15.On the other hand the rod 10 is positioned in the plate 16 such that thefocal line 26 falls in or near the upper surface of the rod, whereby thelongitudinal axis 12 and the line 26 are spaced apart by a distance thatapproximates the radius of the rod 10.

To form a cavity in which the totality of radiation from the diodes(except for losses incurred on reflection) reaches the associated laserelement, the chamber formed by the surface 22 and the plate 16 is cappedby end-members positioned in contact with the front and back surfaces ofthe block 20. Only a portion of the front-end member 28 is shown inFIG. 1. The member 28 contains an opening therethrough (not shown) inregistry with the front end of the laser rod 10. Moreover, the backsurface of the member 28 is coated to be highly reflective to theradiation emitted by the diodes 15. The back-end member (not shown) isfonned and treated in the same way.

In accordance with the principles of the present invention, asemi-elliptical cylinder such as that shown in FIG. 1 is effective toimage the diodes 15 to positions slightly within the rod I0. Only halfan ellipse is needed to accomplish this because each of the diodes l5radiates into a hemisphere with approximately a cosine distribution.Therefore, a linear array of diodes radiates into 180 rather than 360.

Strictly speaking, an ellipse is not an ideal imaging system since onlythe two foci are stigmatically imaged. A bundle of rays from a pointnear one focus arrives in the vicinity of the other focus spread outover an area which depends on the respective angles of the rays of thebundle and the eccentricity of the ellipse.

For an ellipse with no refracting surfaces the distance d of closestapproach to the focus of a ray whose distance of closest approach to theother focus is d is given by where a is the major axis, f the distancebetween the foci, and the angle between the ray and the major axis atits starting point. The quantity d'ld is in effect a magnificationfactor. Since 0 can vary from 0 to 180, this magnification varies from aminimum of af/a+f to a maximum of a+f/af. This magnification dependsonly on the eccentricity e (e =f/a) of the ellipse and approaches one asthe eccentricity approaches zero. The smallest value of f will bedetermined by the diameters of the diode packages and the laser rod.Increasing a will then decrease the eccentricity.

If one makes the major axis 30 in FIG. 1 sufficiently large so thataberrations due to the noted magnification effect are of the same orderof magnitude as the mechanical tolerances of the assemblage, the arrayof discrete diodes 15 will be imaged approximately in the transverseplane (the plane whose normal is, for example, the focal line 24) andspread along the focal line 26.

In accordance with the principles of this invention, the layer 18 on thesurface of the laser rod 10 of FIG. 1 is selected to be highlyreflective to radiation provided by the diodes 15. This layer or coatingmay, for example, comprise a specular or diffuse covering of gold ormagnesium oxide. Only a relatively narrow strip at the top of the rod 10is not so coated. Radiation provided by the diodes l5 enters the rodthrough this uncoated strip. (Advantageously, an antireflective coatingmay be applied to the entry strip to reduce reflection lossestherefrom.)

Diode-provided radiation that enters the rod 10 of FIG. 1 is multiplyreflected therein. Such trapping of the radiation serves tosignificantly increase the efficiency of coupling between the dioderadiation and the absorption states of the rod 10. In one specificillustrative embodiment of the arrangement shown in FIG. 1 wherein thediameters of the emitting areas of the diodes are each approximately0.020 inches and the diameter of the rod 10 is about 0.060 inches, onlyoneninth of the surface area of the rod 10 is not coated with areflective layer. In that embodiment each incident ray that enters theuncoated strip of the rod makes an average of nine traversalstherethrough before exiting from the rod via the uncoated strip.

In FIG. 2 the image 15' of the emitting area of one of the diodes 15 isshown in dashed outline positioned slightly within the surface of thelaser rod 10. It is apparent that the portion of the surface area of therod 10 that is not covered by the reflective coating 18 is determined bythe relatively small diameter of that emitting area. To illustrate themultiple internal reflections experienced by an entering pump ray as aresult of such a large-area coating, one such multiply-reflected ray 29is represented in FIG. 2.

In the specific embodiment discussed above and shown in FIGS. 1 and 2,nine pump diodes are disposed in a linear array. In that embodiment thelaser rod I0 is 2 inches long. The focal lines 24 and 26 are 0.25 inchesapart and the major axis 30 is 2.0 inches long.

In accordance with another aspect of the principles of the presentinvention, a laser rod is pumped in an efficient high power manner bytwo linear diode arrays each positioned in a semi-elliptical cylinder ina spaced-apart relationship with respect to the rod. FIG. 3 shows an endview of such an arrangement.

In FIG. 3 two blocks 30 and 32 are mounted on a base plate 34. The focallines of the semi-elliptical cylinder defined by the block 30 aredesignated by reference numerals 36 and 38 whereas those associated withthe cylinder formed by the block 32 are 40 and 42. A first array 44 ofdiodes is positioned along the focal line 36 to direct radiation at anonreflective longitudinal strip on the upper surface of laser rod 46.Similarly, a second array 48 is positioned along the line 40 to directradiation at a nonreflective strip on the lower surface of the rod. Theremainder of the rod 46 is coated with a layer 50 which is highlyreflective to diode radiation incident thereon.

An end view of the three-dimensional radiation pattern of one of thepump diodes included in the arrangements of FIGS. 1 and 3 is representedby dashed lines in FIG. 4A. As is evident from the pattern, a negligibleamount of the diode radiation is emitted at relatively small or lowangles. Illustratively, the angle A shown in FIG. 4A approximates 20.

Assume that radiation is emitted from spaced points of the emittingregion of diode 52 shown in FIG. 4A. A ray from any such point that isnot located on focal line 54 will be directed to a destination pointspaced apart from focal line 56. Some of these destination points do notlie on the nonreflective strip of laser rod 58. In other words, some ofthe radiation emitted by the diode 52 is not directly reflected into therod 58. This is particularly true of rays emanating from the diode 52that strike surface 60 to the left of center line 62. Of course, some ofthis radiation may subsequently impinge on the nonreflective strip ofthe rod 58 after being multiply reflected from the highly-reflectivesurfaces of the depicted semi-elliptical cavity. But diode-emitted rayswhich are not directly reflected by the surface 60 into the rod 58represent lost pumping power and reduce the overall efficiency of theillustrated structure.

In accordance with the principles of the present invention, thearrangement shown in FIG. 4A may be modified in a unique way to increasethe percentage of the diode radiation that is directly reflected to therod 58. In the modified arrangement shown in FIG. 4B, diode 52 ismounted in a clockwise-rotated tilted fashion. Illustratively, the diode(and hence its radiation pattern, too) is tilted approximately A.Associated rod 58 in FIG. 4B is also tilted in the same direction by thesame number of degrees. Accordingly, a relatively large number of therays emanating from the diode 52 strike the surface 60 in the vicinityof or to the right of center line 62. Because these impact points arecloser to the rod 58 than are the corresponding points in FIG. 4A, theearlier-mentioned magnification or spreading effect caused by thesemi-elliptical reflective surface is minimized. As a result, in theFIG. 4B arrangement more of these emitted rays are directly reflected toimpinge on the nonreflective strip of the rod 58 than in the FIG. 4Astructure. In turn, this makes the pumping efficiency of the arrangementshown in FIG. 4B significantly better than that of the FIG. 4Aarrangement.

In accordance with the principles of this invention, the aforementionedcoupling efficiency can be further enhanced by employing athree-dimensional imaging system to direct diode pump radiation to anassociated laser rod. Either a semiellipsoidal or a hemisphericalreflector may be utilized. In theory, a semi-ellipsoid is preferredbecause for a given overall physical size the aberrations caused therebyare smaller. But, as a practical matter, a hemisphere is preferredbecause it can be fabricated more easily and accurately than anellipsoid. A top view of a specific hemispherical embodiment is shown inFIG. 5.

In FIG. 5 a linear array of diodes 65 and a laser rod 68 are positionedparallel to one another and equidistant from the center 70 ofhemispherical reflector 72. In an illustrative embodiment in which thediameter of the hemisphere is at least eight times the length of the rod68, the depicted arrangement is effective to stigmatically image thediodes 65 to discrete positions slightly within the rod 68. Accordingly,except for discrete areas, it is feasible to coat the entire surface ofthe rod 68 with a high-reflectivity material. Such a coating 74 isindicated in FIG. 5. The large-area coating 74 causes the illustratedembodiment to exhibit a highly advantageous radiationtrappingcharacteristic. As a result, the depicted structure exhibits a highdegree of coupling between the rod 68 and radiation provided by thediodes 65.

Finally, it is to be understood that the above-described arrangementsare only illustrative of the application of the principles of thepresent invention. In accordance with these principles, numerous otherarrangements may be devised by those skilled in the art withoutdeparting from the spirit and scope of the invention. For example, acylindrical (or spherical) refracting element on the nonreflectiveportion(s) of the laser element can be beneficial in collecting andfocusing radiation from the associated spaced-apart pump source to forma more intense image within the element. Additionally, although theprimary emphasis herein has been directed to the use ofrelatively-small-diameter electroluminescent diodes to constitute thepump source, the principles of this invention also extend to the use ofother relatively-small-diameter elements, for example a lamp filament,as the pump source.

What is claimed is:

1. In combination, a laser element, pump means positioned in aspaced-apart relationship with respect to said element, said pump meanshaving an emitting region whose diameter is l/Nth that of the diameterof said element, wherein N is a positive number greater than 1,reflecting means responsive to radiation emitted by said pump means forforming an image of said pump means in the direct vicinity of thesurface of said element, and means disposed on substantially more thanhalf of the surface area of said laser element for reflecting radiationemitted by said pump means and directed into said element, wherebyradiation directed into said element by said pump means is successivelyreflected to make multiple passes through said element before exitingtherefrom, wherein said pump means comprises an array ofelectroluminescent diodes, each of said diodes having an emitting regionwhose diameter is l/Nth that of the diameter of said element, andwherein said pump means further comprises an additional array ofelectroluminescent diodes positioned in a spaced-apart relationship withrespect to said laser element, wherein said means disposed on thesurface area of said laser element covers the entire area thereof exceptfor two opposed relatively narrow longitudinal strips each of whosewidths approximates said emitting-region diameter, and wherein saidreflecting means comprises means defining a distinct semi-ellipticalcylinder for each respective array of diodes for directing radiationemitted by one of said arrays onto one of said longitudinal strips andfor directing radiation emitted by the other one of said arrays onto theother one of said longitudinal strips.

2. In combination, an active laser element, pump means positioned in aspaced-apart relationship with respect to said element, said pump meanshaving an emitting region whose diameter is l/Nth that of the diameterof said element,

wherein N is a positive number greater than 1, reflecting meansresponsive to radiation emitted by said pump means for forming an imageof said pump means in the direct vicinity of the surface of saidelement, and means directly disposed on substantially more than half ofthe surface area of said active laser element for reflecting radiationemitted by said pump means and directed into said element, wherebyradiation directed into said element by said pump means is successivelyreflected to make multiple passes through said element before exitingtherefrom, wherein said pump means comprises an array ofelectroluminescent diodes, each of said diodes having an emitting regionwhose diameter is l/Nth that of the diameter of said element, whereinsaid reflecting means comprises means defining a hemisphere, and whereinsaid means disposed on the surface area of said laser element covers theentire area thereof except for discrete regions corresponding torespective images of said diodes.

i I I k

1. In combination, a laser element, pump means positioned in aspaced-apart relationship with respect to said element, said pump meanshaving an emitting region whose diameter is 1/Nth that of the diameterof said element, wherein N is a positive number greater than 1,reflecting means responsive to radiation emitted by said pump means forforming an image of said pump means in the direct vicinity of thesurface of said element, and means disposed on substantially more thanhalf of the surface area of said laser element for reflecting radiationemitted by said pump means and directed into said element, wherebyradiation directed into said element by said pump means is successivelyreflected to make multiple passes through said element before exitingtherefrom, wherein said pump means comprises an array ofelectroluminescent diodes, each of said diodes having an emitting regionwhose diameter is 1/Nth that of the diameter of said element, andwherein said pump means further comprises an additional array ofelectroluminescent diodes positioned in a spaced-apart relationship withrespect to said laser element, wherein said means disposed on thesurface area of said laser element covers the entire area thereof exceptfor two opposed relatively narrow longitudinal strips each of whosewidths approximates said emitting-region diameter, and wherein saidreflecting means comprises means defining a distinct semiellipticalcylinder for each respective array of diodes for directing radiationemitted by one of said arrays onto one of said longitudinal strips andfor directing radiation emitted by the other one of said arrays onto theother one of said longitudinal strips.
 2. In combination, an activelaser element, pump means positioned in a spaced-apart relationship withrespect to said element, said pump means having an emitting region whosediameter is 1/Nth that of the diameter of said element, wherein N is apositive number greater than 1, reflecting means responsive to radiationemitted by said pump means for forming an image of said pump means inthe direct vicinity of the surface of said element, and means directlydisposed on substantially more than half of the surface area of saidactive laser element for reflecting radiation emitted by said pump meansand directed into said element, whereby radiation directed into saidelement by said pump means is successively reflected to make multiplepasses through said element before exiting therefrom, wherein said pumpmeans comprises an array of electroluminescent diodes, each of saiddiodes having an emitting region whose diameter is 1/Nth that of thediameter of said element, wherein said reflecting means comprises meansdefining a hemisphere, and wherein said means disposed on the surfacearea of said laser element covers the entire area thereof except fordiscrete regions corresponding to respective images of said diodes.